1. Field of the Invention
The present invention relates to a liquid crystal display element.
2. Description of the Related Art
Known are liquid crystal display elements of a twisted nematic (TN) type or a supertwisted nematic (STN) type using nematic liquid crystals. A liquid crystal display element using a ferroelectric liquid crystal is also being proposed. In manufacturing a display element using these liquid crystals, however, it is necessary to apply, for example, an orientation treatment, making it difficult to manufacture a large display element.
Also known is a method of manufacturing a large liquid crystal display element at a low cost without requiring an orientation treatment. For example, a technique of forming a liquid crystal layer by dispersing encapsulated liquid crystal droplets in a polyvinyl alcohol is disclosed in PCT National Publication No. 58-501632 and U.S. Pat. No. 4,435,047. In a display element of this type, the liquid crystal molecules, if they exhibit a positive dielectric anisotropy within a thin film, are oriented in the direction of the electric field in the presence of an electric field. In this case, the liquid crystal layer becomes transparent, if the refractive index n.sub.o of the liquid crystal is equal to the refractive index n.sub.p of the polymer. If the electric field is removed, however, the liquid crystal molecules are brought back to the random arrangement. In this case, the refractive index of the liquid crystal droplet is deviated from n.sub.o, with the result that light is scattered at the boundary region of the liquid crystal droplet and, thus, the liquid crystal layer is turned cloudy.
Additional techniques are known to the art for dispersing a liquid crystal material in a polymer. For example, a technique of dispersing a liquid crystal material in an epoxy resin is disclosed in PCT National Publication No. 61-502128. Also, a technique of dispersing a liquid crystal material in an urethane acrylate resin is disclosed in Published Unexamined Japanese Patent Application No. 2-83534. Further, a liquid crystal display element which permits improving the contrast is disclosed in Published Unexamined Japanese Patent Application No. 63-98631. Specifically, it is taught that an encapsulated liquid crystal portion is provided with a parallel light supply portion and a selective light transmitting portion for improving the contrast.
These polymer dispersed liquid crystal (PDLC) devices are advantageous in that:
1. A bright picture image can be formed because a polarizing plate is not required in the PDLC device;
2. A broad angle of visibility can be obtained in the PDLC device;
3. Since an orientation treatment is not required in manufacturing the PDLC device, it is possible to avoid problems accompanying the rubbing step such as dust generation and generation of electrostatic charges; and
4. The PDLC device is simpler in construction than the conventional liquid crystal display element prepared by sealing a liquid crystal material between a pair of substrates, making it possible to manufacture a large PDLC device without difficulty.
However, several defects are inherent in the conventional PDLC device, as described below: